We're starting to believe those inside sources who say that Leonardo DiCaprio and Blake Lively are "so smitten, it's crazy." How else to explain the starlet's willingness to trek all the way to Australia for a brief and belated birthday celebration with her actor-beau?

Lively turned 24 on Aug. 25, and she marked the occasion by winging Down Under last weekend to see DiCaprio, 36, who is shooting "The Great Gatsby" in Sydney.

One of their first stops: a romantic dinner on Saturday night at hot spot Aria, where they sat at a table overlooking Sydney Harbor.

"She looked gorgeous, and they looked very much like a couple," an eyewitness tells the Daily Telegraph. "They seemed to be having a really nice time and were definitely enjoying the view."

A couple days later, Leo and Blake took a private tour of Featherdale Wildlife Park, where they got up close and personal with the continent's fauna.

"They both loved the koalas and spent quite a bit of time with Austin, one of our younger koalas. Both Leo and Blake were friendly and inquisitive about the animals," their tour guide shares with E! News. "We took them into the dingo enclosure ... They weren't at all afraid of the animals, including the young saltwater crocodile and diamond python Cuddles, who we brought out for them to meet."

The actress, dressed down in jeans, a hat and a floral top, "especially enjoyed meeting the dingoes," continues the staffer, adding that environmentally conscious Leo, wearing his standard baseball cap, asked about the park's endangered species and was treated to a Tasmanian devil feeding.

"They were both so nice and down to earth," the guide gushes to the Herald-Sun. "There was certainly no diva behavior. ... Blake was absolutely beautiful."

And now she's presumably jet-lagged. Lively, whose travels with DiCaprio in recent months have included stops in France, Italy, New York, Los Angeles, Disneyland and Santa Barbara, was back in the U.S. by Tuesday, according to Us.